diff -r 4b64a049f451 Python/formatter_unicode.c
--- a/Python/formatter_unicode.c	Fri Aug 19 12:00:13 2016 +0300
+++ b/Python/formatter_unicode.c	Fri Aug 19 23:22:37 2016 +1000
@@ -32,11 +32,11 @@
 {
     if (presentation_type > 32 && presentation_type < 128)
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '%c'.",
+                     "Cannot specify ',' or '_' with '%c'.",
                      (char)presentation_type);
     else
         PyErr_Format(PyExc_ValueError,
-                     "Cannot specify ',' with '\\x%x'.",
+                     "Cannot specify ',' or '_' with '\\x%x'.",
                      (unsigned int)presentation_type);
 }
 
@@ -105,6 +105,11 @@
     }
 }
 
+/* Locale type codes. LT_NO_LOCALE must be zero. */
+#define LT_NO_LOCALE 0
+#define LT_DEFAULT_LOCALE 1
+#define LT_UNDERSCORE_LOCALE 2
+#define LT_CURRENT_LOCALE 3
 
 typedef struct {
     Py_UCS4 fill_char;
@@ -218,7 +223,13 @@
 
     /* Comma signifies add thousands separators */
     if (end-pos && READ_spec(pos) == ',') {
-        format->thousands_separators = 1;
+        format->thousands_separators = LT_DEFAULT_LOCALE;
+        ++pos;
+    }
+
+    /* Underscore signifies add thousands separators */
+    if (end-pos && READ_spec(pos) == '_') {
+        format->thousands_separators = LT_UNDERSCORE_LOCALE;
         ++pos;
     }
 
@@ -346,11 +357,6 @@
 /*********** common routines for numeric formatting *********************/
 /************************************************************************/
 
-/* Locale type codes. */
-#define LT_CURRENT_LOCALE 0
-#define LT_DEFAULT_LOCALE 1
-#define LT_NO_LOCALE 2
-
 /* Locale info needed for formatting integers and the part of floats
    before and including the decimal. Note that locales only support
    8-bit chars, not unicode. */
@@ -660,8 +666,8 @@
 
 /* Find the decimal point character(s?), thousands_separator(s?), and
    grouping description, either for the current locale if type is
-   LT_CURRENT_LOCALE, a hard-coded locale if LT_DEFAULT_LOCALE, or
-   none if LT_NO_LOCALE. */
+   LT_CURRENT_LOCALE, a hard-coded locale if LT_DEFAULT_LOCALE or
+   LT_UNDERSCORE_LOCALE, or none if LT_NO_LOCALE. */
 static int
 get_locale_info(int type, LocaleInfo *locale_info)
 {
@@ -684,8 +690,10 @@
         break;
     }
     case LT_DEFAULT_LOCALE:
+    case LT_UNDERSCORE_LOCALE:
         locale_info->decimal_point = PyUnicode_FromOrdinal('.');
-        locale_info->thousands_sep = PyUnicode_FromOrdinal(',');
+        locale_info->thousands_sep = PyUnicode_FromOrdinal(
+            type == LT_DEFAULT_LOCALE ? ',' : '_');
         if (!locale_info->decimal_point || !locale_info->thousands_sep) {
             Py_XDECREF(locale_info->decimal_point);
             Py_XDECREF(locale_info->thousands_sep);
@@ -945,9 +953,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;
 
@@ -1092,9 +1098,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;
 
@@ -1270,9 +1274,7 @@
 
     /* Determine the grouping, separator, and decimal point, if any. */
     if (get_locale_info(format->type == 'n' ? LT_CURRENT_LOCALE :
-                        (format->thousands_separators ?
-                         LT_DEFAULT_LOCALE :
-                         LT_NO_LOCALE),
+                        format->thousands_separators,
                         &locale) == -1)
         goto done;